This application is based-on Japanese Patent Application No. 2003-013451 filed on Jan. 22, 2003, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image display apparatus, more particularly to an image display apparatus such as an HMD (head-mounted display) or HUD (head-up display) that projects a two-dimensional image formed, for example, on a liquid crystal display (LCD) onto an observer""s eye by the use of a holographic optical element (HOE) so as to permit the observer to observe the image on a see-through basis.
2. Description of the Prior Art
Japanese Patent Application Laid-Open No. H9-185009, U.S. Pat. No. 4,655,540, and other publications propose image display apparatuses of a see-through type that superimpose an image of an object on an image of the outside world by the use of a combiner realized with a hologram and then projects the resulting image for display. Among these, Japanese Patent Application Laid-Open No. H9-185009 proposes: an image display apparatus in the shape of eyeglasses that permits observation of a two-dimensional image on a see-through basis by the use of a holographic optical element having the function of an eyepiece lens. FIGS. 14A and 14B show an outline of the optical construction of such an image display apparatus. In FIG. 14A, broken lines represent the rays to which a holographic optical element 22 is exposed when it is fabricated, and solid and dotted lines represent the most off-axial rays of the beam representing the reconstructed image when the displayed image is observed.
The holographic optical element 22 is fabricated by the use of a high-coherence light source such as a laser light source. The holographic optical element 22 is fabricated by making a divergent beam emanating from where the observation pupil E is located interfere with a nonaxisymmetric beam with a complicated wavefront and recording the resulting interference fringes as a pattern of refractive index modulation on a photosensitive material. The holographic optical element 22 thus obtained is a volume-phase reflective hologram with good see-through characteristics and high light-use efficiency. This holographic optical element 22 has a phase function that converts the wavefront of incident light in such a way that the light is reflected by diffraction in a desired direction, and also functions as a nonaxisymmetric lens, contributing to miniaturization. When an image is observed, the holographic optical element 22 is reconstructed by the use of a light source such as an LED (light-emitting diode), in particular a light source of which the peak wavelength is roughly equal to the wavelength of the exposure rays (with a difference in wavelength of 20 nm or less). When an image display element 21 is illuminated with the light from such a light source, the image light emanating from the image display element 21 is incident on the holographic optical element 22, and the light reflected by diffraction it exerts is directed to an observer""s eye 23.
The holographic optical element 22 has narrow angle selectivity, and therefore, when it is exposed and reconstructed in the manners described above, its diffraction efficiency is highest when the directions of the reconstruction rays (the solid and dotted lines) are close to those of the exposure rays (broken lines), and is low when the directions of the reconstruction and exposure rays differ. Thus, the uppermost rays represented by the dotted lines and the lower most rays represented by the solid lines come to have a large angle difference xcex4 relative to the exposure rays represented by the broken lines. When photopolymer, a common photosensitive material, is used, the resulting refractive index modulation (in the vicinity of xcex94n=0.1) gives the holographic optical element sharp angle selectivity in terms of diffraction efficiency xcex7. This makes it impossible to obtain an observation pupil E having the designed size. In FIG. 14B, the designed observation pupil E is indicated with solid lines, and the actually obtained observation pupil E is indicated with broken lines. When photopolymer is used, the observation pupil E measures, for example, 10 to 20 mm in the width direction and 1 mm (the broken lines) or 3 to 5 mm (solid lines) in the height direction. As a result, the displayed image goes out of sight when the observer moves his or her eye 23 up or down even a little. Using a photosensitive material such as silver halide or bichromated gelatin instead of photopolymer results in larger values of xcex94n and xcex7, and thus helps obtain broader angle selectivity. This, however, increases fabrication costs and leads to lower durability under the influence of moisture absorption and temperature variation.
An object of the present invention is to provide an inexpensive image display apparatus that forms an observation pupil that permits easy observation of the displayed image.
To achieve the above object, according to one aspect of the present invention, a holographic element is a reflective holographic element formed on a substrate and composed of a composite hologram having a plurality of patterns of interference fringes composed of interference fringes nonparallel to the substrate. This holographic element forms, out of image light having a predetermined wavelength width emanating from an image display element, a plurality of observation pupils at spatially different locations, and acts sin such a way as to fulfill conditional formulae (I) and (II) below for an identical incident ray over the entire area in which the image light is incident on the holographic element:
xcex94xcex8 less than 2xe2x80x83xe2x80x83(I)
xcex94xcex7 greater than 50xe2x80x83xe2x80x83(II)
where
xcex94xcex8 represents the difference (xc2x0) in angle of diffraction among the different patterns of interference fringes; and
xcex94xcex7 represents the difference (%) in diffraction efficiency among the different patterns of interference fringes.
According to another aspect of the present invention, an image display apparatus is provided with an image display element for displaying a two-dimensional image, an enlargement optical system for projecting, with enlargement, the image displayed on the image display element, and a holographic element as described above included in the enlargement optical system.
According to still another aspect of the present invention, an image display apparatus is provided with an image display element for displaying a two-dimensional image, an enlargement optical system for reflecting, as a first image, the image displayed on the image display element so as to project, with enlargement, the first image onto an observer""s pupil while transmitting, as a second image, an image from an outside world so as to direct the second image to the observer""s pupil, and a holographic element as described above included in the enlargement optical system